Conjugated dienes such as butadiene and isoprene tend to be polymerized alone or copolymerized with other copolymerizable unsaturated compound at a step of distillation, extractive distillation, extraction, hydrogen-treatment, hydrogenation purification or heat treatment, or during storage, transportation or processing.
Conjugated dienes are contained, for example, in a C4 hydrocarbon fraction, a C5 hydrocarbon fraction, a hydrocarbon mixture containing hydrocarbon compounds having at least two carbon atoms, such as that produced by removing mathane, hydrogen and nitrogen from a cracker gas from a thermal cracker or a refinery gas from a refinery, a hydrocarbon mixture containing hydrocarbon compounds having at least three carbon atoms, such as that produced by separating and refining ethylene for example, by low-temperature fractional distillation, and a hydrocarbon mixture containing hydrocarbon compounds having at least six carbon atoms, such as that produced by removing a C4 hydrocarbon fraction and a C5 hydrocarbon fraction. In an apparatus for separating and refining these hydrocarbon fractions and hydrocarbon mixtures, a column or tank, a piping, a heat exchanger and the like of the apparatus are liable to be clogged with a polymer deposited thereon. The polymer is believed to be produced by polymerization or copolymerization of the conjugated dienes. Therefore, operation of the apparatus must be regularly or irregularly stopped for cleaning the inside thereof to remove the deposited polymer.
Thus, it is desired to prevent polymerization of conjugated dienes and to minimize clogging of a deposited polymer in an apparatus of separating and refining a hydrocarbon mixture containing conjugated dienes. As the method of preventing polymerization of conjugated dienes, there have bee proposed, for example, a method of distilling a C5 hydrocarbon fraction in the presence of a di-lower-alkyl-hydroxylamine in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. S50-112304, a method of effecting an extractive distillation while furfural and a condensate of furfural are made present in an extraction medium in JP-A S56-81526 or Japanese Examined Patent Publication (hereinafter abbreviated to “JP-B”) No. S43-20281, a method of extractive distilling a conjugated diene hydrocarbon while a polymerization inhibitor or a chain transfer agent is made present in an extraction medium in JP-B S47-41323 or JP-B S45-19682, and a method of prohibiting polymerization of a conjugated diene hydrocarbon by incorporating therein an organic phosphate ester compound in JP-B S49-6886 or JP-S49-7126.
According to the heretofore proposed polymerization-preventing methods and polymerisation inhibitors, even though the polymerization inhibitor is incorporated in a sufficient amount, the problem of clogging in a separating and refining apparatus, especially in a vessel wherein a hydrocarbon mixture is exposed to a high temperature, such as an evaporator or a distillation column, cannot completely be solved.